The present invention relates to a method in the regulation of a multi-layer headbox of a paper machine or board machine. By means of the method and the device in accordance with the invention, it is possible to reliably act upon the grammage profile of the paper across the width of the paper web and also to act upon the fiber orientation profile in the paper web across the width of the paper web. The invention also relates to a multi-layer headbox of a paper machine or board machine.
In a multi-layer headbox, pulps of different sorts in the vertical direction are fed in different layers. One or both of the faces of the paper or board formed out of the jet of the headbox are made representative by using, e.g., high-cost and bleached pulp with a high content of fillers. In a three-layer structure, the middle layer is used to constitute the strength and rigidity of the paper/board, whereas the surface layers hide the less expensive and coarser raw-material in the middle of the structure.
In a multi-layer headbox, when the grammage is regulated conventionally by profiling the shape of the slice, all the layers are affected at the same time, including the covering surface layers. In such a case, the coverage by the surface material is changed in the regulated area and leaves a striped appearance in the product. The profile-bar construction produces turbulence in the jet and deteriorates the purity of the layers.
As is known from the prior art, the direction of the discharge jet of the pulp suspension discharged out of the headbox should differ from the machine direction as little as possible. A directional angle of the discharge jet that differs from the machine direction, which produces distortion of the fiber orientation, has a clear effect on the quality factors of the paper, such as the anisotropy of strength and stretch. The level and variation of anisotropy in the transverse direction also affect the printing properties of paper, such as moisture expansion. In particular, it is an important requirement that the main axes of the directional distribution, i.e. orientation, of the fiber mesh in the paper coincide with the directions of the main axes of the paper and that the orientation is symmetric in relation to these axes.
At the edges of the pulp-flow duct in the headbox, owing to the vertical walls, there is a higher friction. This edge effect produces a very strong linear distortion in the profile. Profile faults in the turbulence generator of the headbox usually produce a non-linear distortion in the profile inside the lateral areas of the flow ducts.
Attempts are made to compensate for an unevenness of the grammage profile arising from the drying-shrinkage of paper/board by means of a crown formation of the slice, so that the slice is thicker in the middle of the pulp jet. It is a phenomenon in the manufacture of paper that when the paper/board web is dried, it shrinks in the middle area of the web to a lower extent than in the lateral areas of the web. The shrinkage is typically in the middle are of the web from about 1% to about 3% and in the lateral areas of the web from about 4% to about 6%. The shrinkage profile produces a corresponding change in the transverse grammage profile of the web so that, owing to the shrinkage, the dry grammage profile of a web whose transverse grammage profile was uniform after the press is changed during the drying so that, in both of the lateral areas of the web, the grammage is slightly higher than in the middle area. As is known from the prior art, the grammage profile has been regulated by profiling the thickness of the jet, either by means of a profile bar construction or by regulating the shape of the discharge duct so that the thickness of the jet is regulated to be larger in the middle area than in the lateral areas of the web.
By means of this arrangement, the pulp suspension is forced to move towards the middle area of the web. However, this circumstance affects the deviation-angle profile of the direction of the discharge jet, which profile further determines the distortion profile of the fiber orientation. The main axes of the directional distribution, i.e. orientation, of the fiber mesh should coincide with the directions of the main axes of the paper, and the orientation should be symmetric in relation to these axes. In the regulation arrangement that profiles the thickness of the jet, a change in the orientation is produced as the pulp suspension flow receives components in the transverse direction.
Regulation of the lip of the headbox also produces a change in the transverse flows of the pulp jet even though the objective of the regulation is exclusively to affect the grammage profile, i.e. the thickness profile of the pulp suspension layer that is fed. Thus, the transverse flows have a direct relationship with the distribution of the fiber orientation.
In the prior art, reference is also made to Finnish Patent Application No. 912230 which describes a headbox that has been divided across its width into compartments by means of partition walls and in which, in an individual compartment, there is at least one inlet duct for the passage of a component flow. Moreover, in the device described in FI 912230, a mixer is connected in front of the individual inlet duct by whose means the pulp suspension ratio can be regulated. In the device of FI 912230, it has, however, not been possible to adequately regulate the mixing ratio without a change in the flow quantity. A detailed device has not been described for carrying out the regulation nor is the device related to a multi-layer headbox.